The present invention relates to an oxygen sensor for sensing an oxygen concentration in gas components of a gas under test such as exhaust gas from an internal combustion engine of an automobile, and more particularly to a high-response oxygen sensor which can prevent the peel-off of an electrode on the side of the sensor which is to be exposed to the gas under test.
A prior art oxygen sensor includes an oxygen concentration sensing element made of an oxygen ion conductive metal oxide such as ZrOz-CaO, and thin film platinum (Pt) electrodes are formed on the surfaces of the oxygen concentration sensing element which are to be exposed to a reference gas and the gas under test, respectively, by chemical plating, vapor deposition or the like.
In the prior art sensor, the oxygen concentration sensing element is highly sintered to form a fine structure because an electromotive force will not be produced if the gas transmits through the oxygen concentration sensing element as the oxygen concentration sensing element is to sense a differential oxygen concentration between the gas under test and the reference gas. As the oxygen concentration sensing element is highly sintered, the adhesion of the electrodes to the sensing element is so weak that when they are left exposed to the gas under test for a long time they are peeled off from the surface of the oxygen concentration sensing element. A difference between thermal expansions of the electrodes and the sensing element also contributes to the peel-off of the electrode.
In order to resolve the above problem, it has been proposed to sand-blast the surface of the oxygen concentration sensing element, which is exposed to the gas under test, to make the surface rough and form fine projections and recesses (namely, to give surface porosity) in order to enhance the adhesion of the electrode and improve the anti-peel-off property of the electrode. However, although the sand-blasted oxygen concentration sensing element has an improved anti-peel-off property, fine cracks are formed on the surface of the oxygen concentration sensor so that the mechanical strength thereof is lowered and the oxygen concentration sensor may be destroyed when it is used in a vibrating environment for a long time. Thus, it has a problem of low durability.
In order to improve the response and the precision of the oxygen concentration sensing element, it has been proposed, as disclosed in U.S. Pat. No. 3,935,089 to form a porous inorganic material on an outer surface of the electrode to carry thereon a catalytic material to promote equilibrium of oxygen partial pressure so that an electromotive force changes abruptly near a stoichiometric air-fuel ratio. Since this type of oxygen sensor is not practical without the catalytic material carried on the outer surface of the electrode, the cost of the sensor is increased when a noble metal such as platinum is used as the catalytic material.
It is an object of the present invention to provide an oxygen sensor having an oxygen concentration sensing element in which a porous film made of a refractory metal oxide is formed on a surface of the oxygen concentration sensing element which is to be exposed to a gas under test and an electrode is formed on the porous film.